Nowadays, in the semiconductor industry, a semiconductor device not only compacts in size as well as light in weight without sacrificing its electrical performance, but also has maximum external I/O connections. As power and performance of semiconductor devices increase, so does the need for maximum I/O connections. One solution to this need is the so-called Ball Grid Array (BGA) semiconductor package.
As shown in FIG. 15, a typical BGA semiconductor package currently used by the industry usually includes a substrate 102 on which a semiconductor chip 101 is mounted. On a top surface of the substrate 102 a plurality of first conductive traces 102b are formed to have electrical connection with pads (not shown) on the semiconductor chip 101 by gold wires 103. A plurality of second conductive traces 102e are correspondingly formed on a bottom surface of the substrate 102 and electrically connect the first conductive traces 102b through plated conductive vias 102c extending through the substrate 102. The second conductive traces 102e on the bottom surface of the substrate 102 each terminate at a conductive pad 102f to form an array of pads 102f on the bottom surface of the substrate 102. A solder boll 104 is attached to each pad 102f on the bottom surface of the substrate 102 to provide external electrical accessibility to the semiconductor chip 101. An encapsulant formed by a molding compound is used to encapsulate the semiconductor chip 101 and the top surface of the substrate 102.
In the molding process for encapsulating a semiconductor chip and the surface of a substrate on which the semiconductor chip is mounted, the mold for molding is required to have a runner connected to a central reservoir for molten molding compound to flow through the runner via a gate into a mold cavity of the mold. Therefore, after the molding is completed, the molding compound in the runner and gate is solidified on the substrate and requires subsequent removal from the substrate. The region of the substrate on which the molding compound in the runner and gate is solidified is usually called "degating region." As the viscosity of the molding compound should be high enough to provide the molding compound sufficient adhesion with the semiconductor chip and the substrate in order to prevent delamitation from taking place, the removal of the molding compound from the degating region on the substrate becomes difficult and often causes damage and deformation to the substrate and the semiconductor package itself.
To solve the aforesaid problem, U.S. Pat. No. 5,635,671 proposes a method for the removal of excess molding compound formed on the degating region of a substrate without damaging the semiconductor device. In the '671 invention, a layer of gold is coated on the degating region of the substrate, allowing the molding compound in the runner and gate to solidified thereon. Therefore, the molding compound in the runner and gate can be easily peeled away from the degating region without damaging or deforming the substrate and the package, for the reason that the adhesive between the molding compound and the gold coating is weaker than that between the molding compound and the substrate. However, as shown in FIG. 16, while the method proposed in the '671 invention is applied to an matrix type substrate sheet for mass production purpose, the width of each connecting portion 112 of the substrate sheet 110 for connecting any two adjacent substrate units 111 is increased for the convenience of applying the gold coating to the predetermined degating region 113 on the substrate sheet 110. Accordingly, the manufacturing cost of the substrate sheet 110 is increased.
Meanwhile, to avoid dislocation of the gold coating over degating region 113 from making the molding compound to directly solidify on areas beyond the degating region 113 of the substrate sheet 110, the application of the gold coating to the degating region 113 has to be well-controllable, thereby it makes the manufacturing cost further increased. Furthermore, gold is a precious metal and can not be retrieved from the substrate for recycling after coated on the substrate. Consequently, it is too costly to use gold as the coating material.
The '671 invention also can not be applied to a BGA semiconductor package having a cavity down structure. As shown in FIGS. 17 and 18, the encapsulant 121 and electrically conductive solder balls 122 are located on the same side of the substrate 120. Electrically conductive solder balls 122 are adhered to the overall top surface of the substrate 120 except the dented region 123 for receiving the chip 124. Thus, runners of a mold are not allowed to pass through the region where conductive solder balls 122 are adhered otherwise after the molding process is completed, the top surface of the substrate 120 is contaminated by the excess molding compound cured in the runners of the mold, thereby adversely affecting the attachment of the conductive solder balls 122 to the substrate 120.